This invention relates to compositions of matter classified in the art of organic chemistry as organic peroxides, more particularly as the peroxide derivatives of the hydroperoxide of 2-methyl-2,4 pentanediol(hexylene glycol hydroperoxide), specifically 4-(t-amylperoxy)-4-methyl-2-pentanol and higher branched chain alkyl analogs, their use in the molecular weight modification of polypropylene and also to articles made from such polypropylene which are suitable for use in regulated food, beverage, pharmaceutical and medical-device applications.
U.S. Pat. No. 3,144,436 teaches the use of organic peroxides for the molecular weight modification of polypropylene by reactive extrusion. A broad class of peroxides suitable for use in the process is defined in terms of a broad range of half-life temperatures and injection of solvent solutions of a peroxide into the melt zone of the extruder is the preferred method of operation. The broad class of peroxides defined does not enable one to select a peroxide which does not generate or only generates small amounts of t-butanol in practice, which can be used without solvents and which otherwise minimizes to an acceptable level safety hazards inherent in handling organic peroxides.
Over time, because of its safety in handling and decomposition temperature, one specific peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (LUPEROX(copyright) 101), has become the industry standard for polypropylene modification.
While generally satisfactory from the viewpoint of efficiency, of being useable without solvent dilution and from the safe handling standpoint, this product is known to leave an amount of residual t-butanol in the extruded polypropylene which meets food grade standards but which for some uses is objectionable. Therefore, a substitute having similar safe handling without the need for solvents, roughly the same efficiency in actual use in the extrusion process for polypropylene molecular weight modification and which does not produce objectionable amounts of t-butanol as a decomposition product has long been sought by the industry.
An attempt to address this issue is shown in U.S. Pat. No. 4,707,524 which discloses the use of peroxides which do not decompose to t-butanol and which have a half-life in the range from about 1.0 to 10 hours at 128xc2x0 C. for modification of the molecular weight of polypropylene. As shown by the actual data in the tables in that patent, however, LUPEROX 101 was far more efficient than either the 2,2 di(t-amyl)peroxy propane (LUPERSOL(copyright) 553) and 3,6,6,9,9-pentamethyl-3-n-propyl-1,2,4,5-tetraoxacyclononane (ESPERAL(copyright) 529) actually compared with it. The discussion of the background art at columns 2 and 3 of this patent may also be of interest in understanding the present invention.
EP 0853090 Al discloses the use of di-t-amyl peroxide as suitable for polypropylene molecular weight modification while avoiding the generation of t-butanol. However, di-t-amyl peroxide suffers from a very low flash-point that compromises safety and handling characteristics in polypropylene modification processes.
The majority of today""s production processes require that the peroxide be mixed with solid polypropylene in a blender. Under such conditions, it is crucial that the peroxide have a high flash-point for safety. The flash-point of di-t-amyl peroxide is roughly 25xc2x0 C. while the flash-point of LUPEROX 101 ranges from about 50xc2x0 C. to 75xc2x0 C. depending on quality. 4-(t-Amylperoxy)-4-methyl-2-pentanol also has a flash-point greater than 50xc2x0 C. and can be used in similar fashion to LUPEROX 101 in current mixing equipment.
4-(t-Amylperoxy)-4-methyl-2-pentanol is among the hexylene glycol-derived perester and peroxide compounds generically disclosed in U.S. Pat. No. 3,236,872.
The compounds are stated to be useful in the crosslinking of substantially saturated polymers including polypropylene but no t-amyl derivatives were actually tested and crosslinking of polypropylene would increase molecular weight and is the exact opposite of the molecular weight reduction and variation narrowing which occurs during the normal polypropylene molecular weight modification process (lower molecular weight is equivalent to faster melt flow in viscosity measurements).
4-(t-Amylperoxy)4-methyl-2-pentanol has over the years found utility as a reactant or a reaction catalyst which made use of its hydroxy functionality for various purposes. Illustrations of these uses are contained in: U.S. Pat. Nos. 5,475,072; 5,494,988; and 5,489,699; Laerdere et al, Ann. Tech. Conf.xe2x80x94Soc. Plant. Eng. (1998); and Callais, Proc. Water-Borne High Solids Coat. Conf. (1990).
Nothing in any art known to applicants suggests that 4-(t-amylperoxy)-4-methyl-2-pentanol will have activity, on an equal active oxygen basis, approximating the efficiency of LUPEROX 101 in the modification of polypropylene while generating commercially acceptable levels of residual t-butanol in the modified polypropylene.
One other recent reference, U.S. Pat. No. 5,932,600 (and its related U.S. and European counterparts) teaches the use of cyclic ketone peroxides particularly methyl ethyl ketone cyclic trimer peroxide for polypropylene modification. The principal advantage of these products is that they do not produce t-butanol as a decomposition by-product. However, these peroxides suffer from excessively long half-lives and the required use of safety diluent. The long half-life is undesirable because it will lead to product quality problems (residual peroxide in resin) or lower productivity/higher resin color depending on the production changes made to avoid undecomposed peroxide in the resin (longer residence times or higher temperatures in the extruder). In addition, diluents are undesirable in at least some polypropylene grades because they may produce xe2x80x9csmokingxe2x80x9d or xe2x80x9cdrippingxe2x80x9d in an end user""s extruder. It has been reported that diluents are also undesirable for fiber or film grades where, for example, they may adversely affect the feel of the surface.
The invention provides in a first process aspect, a process for the molecular weight modification of polypropylene which comprises treating polypropylene with a molecular weight modifying effective amount of 4-(t-amylperoxy)-4-methyl-2-pentanol for a time and a temperature sufficient to induce decomposition of the 4-(t-amylperoxy)-4-methyl-2-pentanol and thereby modify the molecular weight of the polypropylene.
The first process aspect of the invention provides a process for the conversion of very high molecular weight, difficult to process polypropylene to low or moderate molecular weight, easy to process polypropylene at an efficiency approximately equal to the conversion efficiency of LUPEROX 101 at approximately equivalent active oxygen levels without the generation of commercially unacceptable levels of t-butanol, while minimizing the hazards such as those incurred in handling di-t-amylperoxide, the need of safety solvents or modifying process conditions to accommodate higher half-life initiators.
The invention provides in a first composition aspect, a non-toxic polypropylene containing composition adapted for the handling and packaging of foods, beverages, or pharmaceuticals, or for use in a medical devices comprising a polypropylene resin which has been molecular weight modified by the first process aspect of the invention and which contains less than 100 ppm (parts per million) t-butanol.
In practicing the processes of the invention to prepare the melt flow modified polypropylene resulting from their practice, conventional, well known, procedures for incorporating the peroxy compound into and reacting it with the polypropylene may be employed. These techniques are described in the previously cited references and no particular technique is considered particularly critical to the practice of the invention.
Conveniently, known amounts of peroxide are premixed with polypropylene flakes, powders or pellets containing conventional additives and/or stabilizers, preferably under an inert atmosphere (absence of molecular oxygen). The polypropylene contemplated as being modified by the invention includes copolymers with up to about 25% by weight ethylene. The peroxide material should be added to the polypropylene, pellets, flake or powder in concentrations of from 50 to 10,000 ppm by weight (molecular weight modifying amount). More desirable is from 100 to 2,000 ppm of peroxide. The components (polypropylene, peroxide and additives) may be premixed at room temperature or above and then in an extruder at temperatures not exceeding 550xc2x0 F. (about 288xc2x0 C.), or more desirably from 200 to 260xc2x0 C., or the polypropylene powder, pellets or flakes and additives can be premixed at room temperature and fed concurrently with peroxide to an extruder, or all the ingredients can be preblended in a heated mixer, not exceeding 100xc2x0 C. prior to adding to an extruder.
The mixture should be processed at a temperature of from about 350xc2x0 F. (177xc2x0 C.) to 550xc2x0 F. (288xc2x0 C.) (temperature sufficient to induce decomposition of the 4-(t-amylperoxy)-4-methyl-2-pentanol) for a time necessary to reduce the melt flow rate to the desired rate (which may be readily determined by a few pilot experiments by one of skill in the art). More particularly the present invention contemplates as a particular advantage its use at time and temperature profiles currently employed for LUPEROX 101, generally temperatures less than 240xc2x0 C.
When 4-(t-amylperoxy)-4-methyl-2-pentanol is used in the above general procedure, no peroxide material remains in the polypropylene, and the amount of t-butanol contained in the peroxide is substantially less than the residual concentration left by LUPEROX 101 when used on an equal active oxygen basis.
Although the invention has been illustrated by the addition of the peroxide to the modification process as a master batch absorbed on polypropylene, one of skill in the art will understand that it may be used as a neat liquid injected directly into an early stage of the extrusion process, or as a master batch absorbed on an alternative, convenient carrier material.
Other materials contemplated as equivalent in the process and practice of the invention are: dihexylene glycol peroxide; 4-(t-hexylperoxy)-4-methyl-2-pentanol; 4-(t-octylperoxy)4-methyl-2-pentanol; 2-methyl-2-t-amylperoxy-4-pentanone; di-t-hexyl peroxide; di-t-octyl peroxide; the t-amyl, t-hexyl and t-octyl analogs of LUPEROX 101; mixed dialkyl peroxides such as t-amyl-t-hexyl peroxide and t-amyl-t-octyl peroxide; and mixtures thereof.
The following examples further illustrate the best mode contemplated by the inventors for the practice of their invention and should be construed as illustrative and not in limitation thereof.